Electron microscope



June 10, 1947. 5, RAMO 2,422,041

ELECTRON MICROSCOPE Filed Feb. 18, 1944 Inventor: Sim on Ram o,

His Attorneg.

Patented June 10, 1947 General Electric Com New York pany, a corporation of.

Application February 18, 1944, Serial No. 522,875

My invention relates toelectron microscopes and has for its object to provide a new and improved-method and apparatus for obtaininghighly enlarged. images of very'small structures by: the use of electrons.

Itis-known that surfaces of hot, cathodes may be studied by accelerating. electrons emitted by that cathode to force them to strike a fluorescent screen. When used in conjunction withproper shaping. of accelerating fields, either electric, magnetic, or a combination of both; the image may be made larger than the cathode. It is known also that, if a conductive wire is drawn to a very-fine tipand etched'down smoothly electrons may be drawn. from the tip or point by field emission to travel essentially inradial: lines. With such fine points, however, it is necessary to use low voltages between the point" and an anode structure to prevent injury to thepoint resulting. in inability of low velocit electrons to penetrate even the thinnest samples placed between the point and the electron screen without serious scattering of the electrons.

It is a further object ofmy invention to-provide. a new and improved electron microscope structurev which permits the use of a cold cathode and relatively low voltages to provide a shadow image of a specimen to be observed.

It is another object of my invention to provide a new and improved electron microscope which employs solely a. cold cathode structure and an anode or fluorescent screen.

It is a further object of my invention. to=provide. a new and. improved electron microscope which requires but a smallvacuum. chamber and relatively poor vacuum conditionsfor. satisfactory operation.

It is a still further object of my invention to provide anew and improved electronmiscroscopy technique in whicha cold-cathode is employed as a support for a specimen to be observed.

It is still another object of my invention to provide a new and improved electron microscope in which the. cathode and anode or fluorescent screen are spaced apart by extremely small distances.

It is a stillfurther object of m invention to provide a new and improved electron microscope relatively simple in structure. and characterized by its small size and low cost.

One of the features of. my invention is theuse in an electron. microscopy of. the. shadow microscopy typeof a cold cathode upon which is sup ported aspecimen to beobserved and ananode. or fluorescent screen closely spaced with respect to 7 Claims. (01. 250-495) said cathode. A relatively low potential im pressed between the cathode and anode produces electron emission solely through. the actionof the electric field existing between these electrodes, the electrons being drawn from the portion of the cathode not covered by the small specimensto produce anenlarged shadow image of the specimen on the fluorescent screen.

The features ofthe invention which I desire to protect hereinare pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by referring. to the following description takenin connection with the drawing, in which Fig. 1 represents.diagram matically and in section an electron microscope; Fig. 2 illustrates an enlarged view of a portion of-the cathode. structure employed intthe. electron. microscope of Fig. 1; and Fig. 3 illustrates a modification of an electron microscope employing an alternative energizing potential arrange,- ment.

Referring particularly to Fig. 1, there is shown an electron microscope comprising a closed container I0 of tubular construction. At one end, the container is. closed by a glass window ll having a fluorescent screen or material l2 on its inner surface, upon which is deposited as by evaporation a very thin film l 3 of a suitable metal suchv as for example aluminum. At the opposite endof the container... there is supported a cathode l4 so constructed as to. be easily inserted and withdrawn from the container. The container l0 may be formed of glass and may have at its cathode end. a flanged member l5 formed of a suitable alloy,.such asan iron-nickele-cobalt alloy, for uniting with the glass container Ill. To' this end, the container H) may be formed of a borosilicate glass which unites readily with the alloy material l5. The anode M' may be provided with a circular metallic ringlfi at" its end external to the container [0 which is in opposed relation with the member l5; being secured thereto by a plurality of bolts IT. A gasketor sealing material It may be interposed between the members I5 and 16 to provide an air-tight seal aroundthe cathode opening of the container. The enclosure Hlmay be evacuated by any-suitable means, such as a vacuum pump, attached to an outlet tubulation IS.

The? cathode 14- preferably is formed of asuit.- able metallicmaterial, such as tungsten, which is provided, at its inner extremity with: a smoothly rounded tip-or point 20. The tip 20 may. be produced by etching a tungsten wire until the. tip

is spherical in character having a very small radius of the order of 10- centimeters or less. The tip 20 preferably is spaced relatively close to the anode or fluorescent screen I 2, the distance .between tip 20 and screen I2 being of the order of a few millimeters. Because of the miniature structure of the cathode and the small distance between the cathode and the fluorescent screen, the container l may be of very small size, the enclosure having a volume of only a few cubic centimeters.

In the enlarged view of the tip 20 of the cathode shown in Fig. 2, the tip 20 is covered with an adhesive or plastic film 22 in which is embedded particles of very small structure or specimen 23 to be observed. Referring again to Fig. 1, an electric field is established between cathode l4 and fluorescent screen l2 by any suitable means, such as battery 24, the positive terminal of which is connected to ground and to the metallic coating I3 which covers the fluorescent screen 12. The negative terminal of the battery is connected. to the cathode I4 through any suitable energizing means, such as the switch 25.

In the operation of the miniature two-element electron microscope thus described, since the cathode is placed relatively close to the anode and the container ID has a relatively small volume, a rough vacuum condition within the container I0 is sufficient for shadow microscopy work. The cathode I4 is of the cold cathode type and electrons are drawn from the point 20 solely by field emission and travel essentially in radial lines to the anode or fluorescent screen l2. The radius of the tip 20 is sufhciently small and the surface of the tip is sufiiciently smooth that the electrons appear to come from a point very close to the surface of the tip. The potential applied between the cathode and anode is held to a low value, of the order of 4 or 5 kilovolts or less, bein sufficient merely to provide field emission from the cold cathode tip. At this potential, there is no question of the electrons penetrating the specimen 23 on account of their low velocity. The electrons are emitted from the surface of the tip 20 which is not covered by the specimen, i. e they flow through the holes in the structure which covers the tip. Since the electron flow is of the rectilinearly accelerated field emission type, it creates an image of the surface of the tip 20, the specimen 23 participating as a part of the image. As a result, an enlarged shadow image of the specimen is produced. on the fluorescent screen. 1

One of the advantages of the electron microscope illustrated in Fig. 1 is that a cold cathode is employed which may be easily positioned within and withdrawn from the microscope and the specimen which is placed upon the cathode is not subjected to the high temperature conditions encountered in cathodes of the thermionic type. Furthermore, since electron emission is of the rectilinearly accelerated field emitted type, the

resolution contained in the projected shadow of the specimen placed on the cathode is very high. Another advantage is that the relatively low voltage and close spacings employed assure stable performance without serious pitting of the smoothly etched cathode tip, which pitting would result in loss of rectilinear motion of the electron flow and the resolution of the microscope.

Another important advantage of the electron microscope thus described is the absence of all electrostatic and magnetic lenses with their comwithout adhesive film 22, the specimen material may become charged due to the electron emission of the cathode surface between the small specimen particles, and the specimen may show a tendency to fly off of the cathode surface due to repulsion of like charges, as Well as the attraction of the negatively charged particles to the positively charged anode. For this reason, the field applied between the screen 12 and the cathode i4 is pulsed repeatedly, the pulse being of the character indicated as applied between the terminals 30, 3!, and comprises a pulse of relatively high negative potential 32 applied for a short period of time followed by a pulse of less negative potential 33, which may be of zero or positive potential and which is applied for a longer period of time. The duration of the pulse 32 is made shorter than the charging time of the specimen particles on the cathode tip. The duration of the potential 33, when the potential is almost zero or slightly positive, is long enough to allow discharge of the specimen particles. Use of a pulse of this type assures operation of the electron microscope without displacement of the specimen on the cathode tip. For particles which are quite readily charged and in which this displacement occurs, the film 22 shown in Fig. 2 may be employed, the thickness of this film being of the order of angstrom units, as compared with the radius of the spherical tip 20 which is of the order of 10,000 angstrom units.

From the above description, it is seen that my invention provides a two-element electron microscope which is extremely small in size and of relatively simple construction. The use of a cold cathode and an anode as the sole electrodes of the microscope obviates the requirement of large size and the 'difiiculty of insulation of a lens system. By using shadow microscopy technique and employing potentials such that electrons will not penetrate the specimen itself, injury to sensitive specimens is avoided and clouding of the image on the fluorescent screen, due to scattering of electrons which occurs when electrons pass through the specimen itself, does not occur, with the result that a high resolution of the projected shadow of the specimen is obtained.

While I have shown a particular embodiment of my invention, it will of course be understood that I do not Wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover any such modifications as fall Within the true spirit and scope of my invention.

What I claim as new and. desire to secure by Letters Patent of the United States, is:

1. The combination, in an electron microscope, of a cold cathode structure comprising a metallic member having a smoothly rounded tip adapted to support a specimen to be examined, an electron sensitive plate mounted in spaced relation with said tip, and means for establishing an electric field between said cathode and said plate whereby electrons are emitted from said tip to produce on said plate a shadow image of a speci men supported on said tip.

2. An electron microscope comprising an evacuated chamber, a cathode supported in said chamber comprising a metallic member having a rounded tip adapted to support a specimen to be observed, a fluorescent screen supported adjacent said tip, and means for establishing an electric field between said cathode and said screen to produce electron emission from said tip, said electrons being emitted from the portions of said tip not covered by a specimen supported on said tip to produce on said screen a shadow image of a specimen so supported.

3. An electron microscope comprising a cathode constituted by a metallic member having a rounded point, an adhesive film covering the surface of said point and adapted to have embedded therein a specimen to be examined, an electron sensitive screen, and means for establishing an electric field between said cathode and said screen to produce an emission of electrons from the portions of said point not covered by a specimen whereby a shadow image of a specimen may be produced on said screen.

4. The method of operating an electron microscope having a cathode and a fluorescent screen in spaced relation which comprises coating said cathode with an adhesive material, supporting specimens to be observed from said material, and establishing an electric field between said cathode and said screen to produce electron emission solely from the action of said electric field and to obtain a shadow image of said specimen on said screen.

5. The combination, in an electron microscope, of an evacuated chamber, means for supporting a cathode and a fluorescent screen in spaced relation within said chamber, said cathode comprising a metallic member having a smoothly rounded tip at its end adjacent said screen, a thin film of plastic material covering said tip and adapted to have embedded therein a specimen to be observed, means for establishing an electric field between said cathode and said screen to produce emissions of electrons from the portions of said tip not covered by a specimen whereby a shadow image of a specimen may be produced on said screen.

6. The combination, in an electron microscope, of a cold cathode comprising a metallic member having a smoothly rounded end for supporting a specimen to be observed, a fluorescent screen in spaced relation with said end, and means for establishing an electric field between said screen and said cathode, said means comprising means for providing a pulse of negative voltage to said cathode of relatively short duration and a pulse of positive voltage for suflicient duration to permit discharging of a specimen supported on said end to remove electrons collected thereon during periods of said negative pulse.

7. The combination, in an electron microscope, of a cold cathode comprising a metallic member having a smoothly rounded end for supporting a specimen to be observed, a fluorescent screen in spaced relation with said end, and means for establishing a cyclically varying electric field between said screen and said cathode, said means comprising means for providing a pulse of negative Voltage of relatively short duration for a portion of each cycle, and means to permit discharging of a specimen supported on said end during another portion of each cycle to remove electrons collected thereon during said portion of said negative pulse.

SIMON RAMO.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,363,359 Ramo Nov. 21, 1944 2,271,990 Ramberg et a1. Feb. 3, 1942 2,249,453 Boersch et al July 15, 1941 2,274,215 Ruska Feb. 24, 1942 OTHER REFERENCES Applied Engineering, published -'by John Wiley 8: Sons, Inc., New York. Copyright 1943. Copy in Division 54. 1

Myers, Electron Optics," pages 441, 446, 447. Published 1939 by Van Nostrand 00., 250 Fourth Ave., New York city. Copy in Div. 54. i 

